Most analytical instruments require some degree of calibration to assure the reliability of their measurements and results. Generally, most such instruments undergo this calibration through the use of a standard sample having a known value of the parameter under study by that instrument.
Some analytical instruments only require very infrequent calibration. In fact, the calibration effected upon the building of the instrument may suffice for all of its life in normal wear. Maintaining the instrument in calibration, clearly, does not present a major problem for the instrument's user.
Other instruments do not share this good fortune. The components establishing the correct parameters for proper performance of analyses possess varying degrees of instability that cause them to change over time. Merely turning the instrument's power off and on may effect an unacceptable degree of change. Accordingly, many instruments require relatively frequent calibration to provide meaningful results.
The problem of frequent calibrations becomes even more severe where relatively unsophisticated personnel operate the equipment. The problem becomes particularly egregious where the tests have medical origins. Any inaccuracy in the results due to improper calibration can deleteriously seriously affect an individual's health.
Accordingly, many instruments have attempted to incorporate some aspect of simplified or even partially automated calibration. A first problem in automated calibration, however, involves the attempt to find the exact peak for a physical event undergoing study. In fact, locating the center of the peak becomes excrutiatingly difficult when under non-human control. Particularly does this occur where more than a single phenomenon provides a double peak or a peak with a shoulder.
A second difficulty with automated calibration involves performing the task within a reasonable amount of time. The instrument must scan a large range of possible values of the involved parameter in order to find the peak. Performing the process rapidly results in spending very little time at each value of the parameter and thus leaves a large uncertainty in the results. Taking a long time to conduct the analysis to achieve the requisite accuracy often results in an extraordinarily slow process taking an unacceptable amount of time.
U.S. Pat. No. 4,060,726 to S. H. Luitwieler et al. shows a scheme which attempts to calibrate an instrument over several different energy ranges. The patent employs two PHA's. It sets one with a large window to cover a large range below the desired energy setting and the second with a large window to cover the range above the desired setting. The scheme then employs coarse adjustments in an effort to bring the observed peak near the desired setting.
Luitwieler et al. then reduce the magnitude of the voltage adjustments in successive steps until they get to the lowest increment available. This brings the instrument somewhere in the range of the peak. It then decreases the size of the window, discards the second PHA, and utilizes a longer count period. The scheme then increases and decreases the voltage settings by the smallest available amount in an effort to find the maximum count and presumably the center of the peak.
The Luitwieler et al. scheme requires two PHA's, several adjustments in the windows and voltages, and results in a degree of uncertainty for a sample that does not have a perfectly sharp peak. Accordingly, it does not solve the two problems associated with automatic calibration discussed above.